This invention relates to a method of drilling dovetail pins, as well as auxiliary and cross keys installed in turbine rotors, to facilitate removal thereof.
Certain steam turbine buckets utilize a finger dovetail configuration to provide attachment of the buckets to the turbine rotor. Specifically, generally radially oriented fingers on a bucket mate with opposite mating fingers machined in the rotor. Each bucket dovetail has three axially extending dovetail pin holes that may be aligned with corresponding holes pre-drilled in the rotor. After the buckets are fit to the rotor, the dovetail pin holes in the buckets are aligned with the corresponding holes in the rotor. After all of the buckets have been installed, the pin holes are final-reamed to allow the installation of the dovetail pins that hold the buckets in place on the rotor. During operation in a severe environment, the dovetail pins may become stuck in the bucket and/or rotor pin holes due to pressure and temperature effects, oxidation, as well as stepping of the pins due to the buckets pulling radially on the pins as a result of centrifugal forces. Removal of these dovetail pins for inspection or replacement can be difficult, time consuming and costly.
Conventional methods of removing dovetail pins involve using a jack, a peening gun, and/or a powder actuated gun to push the pins out of the holes. If the pins do not move, and in the event the buckets were to be scrapped, one option has been to cut off the main section of the buckets and gang mill out the dovetail portion of the buckets from the rotor fingers. Residual material was removed by hand. If the buckets were to be saved, however, the dovetail pins were drilled through, using conventional manual drilling methods, and the leftover shells were removed again by hand. Another method used was to drill the pins part way through, insert a driver rod, and then push the pins out with a peening gun or powder actuated gun. Additional problems with the prior drilling methods involve: (1) a tendency for conventional drill bits to xe2x80x9cwalkxe2x80x9d off center when drilling through a hard pin, and subsequently extend through the side of the pin and into the rotor; (2) the extensive time involved in hand picking out shells left when pins are drilled; (3) if the drilling is completed too deep into the pin, the driving operation can separate the end of the pin, leaving a partial shell in a hole which needs to be removed by hand; (4) when the pins are drilled, any initial misalignment of the drilling machine can also result in the drill bit or EDM electrode extending through the side of the pin and into the rotor. The deeper the drilling, the more profound the effect of misalignment and the greater the chance of rotor damage.
The present invention seeks to eliminate the above described problems associated with prior drilling methods. In the exemplary embodiment, a step drilling procedure is utilized which involves drilling a relatively large diameter hole (preferably using rapid EDM drilling) in the pin to be removed, for approximately ⅓ of the pin length. The electrode is then removed and pin removal is attempted via conventional means. If the pin does not move, the electrode is replaced with a smaller electrode and a smaller diameter hole is continued to a point beyond the midpoint of the pin. After the electrode is removed, pin removal is again attempted. If the pin still does not release, a final drilling step utilizes the same smaller electrode of the previous step to continue to drill the hole to about xc2xe of the length of the pin. Testing has demonstrated that at this point, the pressure on the pin will be sufficiently relieved to enable removal of the pin. It is significant that the step down in electrode size while drilling deeper, accommodates any slight misalignment of the electrode, reducing the potential for drilling through the side of the pin and into the rotor.
Accordingly, in its broader aspects, the present invention relates to a method of removing a pin from a hole comprising a) drilling a first hole of a first diameter in the pin a selected fraction of a length dimension of the pin and attempting removal of the pin; b) if the pin cannot be removed, drilling a second hole of a second diameter smaller than said first diameter an extended fraction of the length dimension of the pin; and c) removing the pin.
In another aspect, the invention relates to a method for removing a dovetail pin from a turbine bucket-to-rotor finger dovetail arrangement wherein generally radially oriented fingers on the bucket and on a rotor wheel are interleaved and where a plurality of axially oriented dovetail pins secure the buckets to the rotor, the method comprising a) drilling a first hole in the pin of a first diameter and extending about ⅓ of an axial length dimension of the pin; b) drilling a second hole, continuing from the first hole, of a second diameter less than the first diameter, and extending about xc2xd the axial length dimension of the pin; c) drilling a third hole, continuing from the second hole, of a diameter substantially equal to the second hole, and extending about xc2xe the axial length of the pin; and d) removing the pin.